The present invention relates generally to an illumination optical system for endoscopes, and more particularly to an illumination optical system for endoscopes that is used in opposition to a light beam exit end of a light transmission means for transmission of light from a light source and that is designed especially for a slimmed-down endoscope.
For a scope used for endoscopic inspection, there has been mounting demand toward a wide-angle field of view in view of prevention of making an oversight. In recent years, some scopes having a full angle of view from 140° to 170° have been available. To obtain wide-angle subject images, it is indispensable to make the angle of field of an objective lens wider; however, if the ability of an illumination optical system to distribute light to the periphery of the field of view is worse, an effective viewing angle then becomes narrow too. Accordingly, when it comes to making the field of view of an endoscope wider, the ability of the illumination optical system to distribute light over a wider range must be factored in.
On the other hand, much is still left to be desired about the diameter of the scope itself for the purpose of easing off pains to patients. The requirements for such an illumination optical system as used on endoscopes are that the distribution of light to around the field of view be satisfactory and, moreover, the diameter of an illumination lens be small. The situations being like this, for an illumination optical system adapted to diverge rays just emitted from a light beam exit end via a negative lens as shown typically in Patent Publication 1, it is required to make the diameter large, thereby preventing the ability to distribute light from going worse by shading. As a result, the proportion of the area of the illumination lens in the scope's leading end grows high, rendering diameter reductions difficult. As shown in Patent Publications 2 and 3, there are some attempts at improving the ability to distribute light using a less costly member such as an inexpensive-to-fabricate transparent sphere. Nonetheless, there are still similar problems remaining unsolved because they take aim at wide light distribution by divergence using a negative lens as is the case with Patent Publication 1.
The situations being like such, most illumination optical systems for endoscopes make use of positive lenses, as exemplified in Patent Publications 4 to 9. Once a light beam has been collected in the illumination optical system, it is turned into divergent light for illumination. By doing this, diameter reductions are achievable while the ability to distribute light is improved, thereby making the diameter of the scope small.
And now, it is general that subjects viewed mainly through endoscopes are not always of constant shape. There are many subjects such as the wall of the abdominal cavity that is relatively close to a plane, the internal wall of the digestive tract in a tract and cavity form, the cardiac region or its vicinity that is close to spherical shape, or the like.
For instance, when a planar subject is viewed, it is well known as the cosine forth law that as an aberration-free optical system having limited quantity losses is used, it causes a relative illuminance distribution to decrease in proportion to cos4θ. Accordingly, the use of this for illumination would cause the periphery to get dark: this is the reason the aberration-free lens (f·tan θ lens) is not suited for illumination. To obtain uniform illuminance over the field of view in the case of a planar subject, it is preferable to use a regular projecting lens (f·sin θ lens), as set forth in detail in Patent Publication 4.
When a curved subject such as a spherically shaped subject is viewed, on the other hand, it is preferable to use an equidistant projecting lens (fθ lens) that satisfies h=fθ where h is the height of light incident onto the illumination optical system, θ is an exit angle, and f is the focal length of the illumination optical system. With this, it is possible to obtain uniform illuminance on the spherical surface and so obtain an image that is easiest to view, as set forth in detail in Patent Publication 5.
What is common to these is that to improve light distribution capability using the illumination optical system, large distortion must be produced.
Of course, if an aspheric surface is used as shown in Patent Publications 4, 5 and 6, it is then possible to produce large distortion, thereby achieving wide light distribution. However, the angle of field of the objective lens grows wide, and if it is intended to obtain wide light distribution over a wider range, it is then required to configure the lens into a steeper convex shape. As a result, power grows strong and so high precision is in demand. This leads to a problem: cost rises. The aspheric surface is processed mainly by molding at the cost of the degree of freedom in selection of materials, because the glass material capable of molding is limited. In other words, fine adjustment cannot be implemented by material replacement.
In addition, the above theory should be applied to where there is none of losses caused by shading of rays in an optical path: the fact that the above illumination optical system is idealized too far must be taken into account. That is, especially with an endoscope illuminating optical system that places weight on fineness, care should also be taken of how light quantity can be delivered with no quantity losses. In other words, light rays are indeed shaded by a frame or the edge of the lens en route to the periphery of the field of view: it is difficult to deliver light rays up to the periphery of the field of view. For instance, referring to such an optical system as shown in Patent Publications 7, 8 and 9, the applicable angle of field is barely about 120° at most; in other words, with a scope of 140° or greater, it is difficult to improve light distribution.
There are two reasons: (A) distortion produced at the illumination optical system remains small so that light is likely to be collected at the center, making light quantity at the periphery smaller than that at the center; and (B) when it is intended to produce large distortion, light leaving parallel with an optical path is likely to be shaded on the way to the periphery of the field of view, resulting in efficiency losses of the illumination system. For two such reasons, there can be no tradeoff offered between efficiency and light distribution. In addition, as the diameter of the scope gets smaller, influences of decentration, if not large, grow larger. For instance, there is a phenomenon where illumination light is locally biased. Accordingly, strict tolerance must be applied to parts, and processing costs rise as well.
Patent Publication 1: JP(A) 2003-131144
Patent Publication 2: JU(A) 5-94835
Patent Publication 3: JP(A) 2005-304838
Patent Publication 4: JP(A) 2003-5095
Patent Publication 5: JP(A) 5-157967
Patent Publication 6: JP(A) 6-148519
Patent Publication 7: JP(A) 8-320440
Patent Publication 8: JP(A) 2000-275547
Patent Publication 9: JP(A) 2002-182126